Simplified two-coordinate electro-optic prism deflector

ABSTRACT

There is disclosed an arrangement of two-coordinate deflectors without the 90* polarization rotator between the stages that was previously thought to be essential. This reduction in the number of optical parts is achieved by mutually rotating only the inclined prism interfaces in the successive stages, while maintaining the crystalline axis orientations and electrode orientations the same in the two stages. The new deflector is easier to align and scatters less light than prior prism deflectors; and it will be useful in multiple-pass light deflectors and optical time-division multiplex communication systems.

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[72] Inventors Edward A.Olll [56] References Cited "01mm UNITED STATESPATENTS Ralph I". Tramberulo, Red Ink, NJ. [2n APPL No. 858,7083,439,975 4/1969 Smith 1. 350/150 [22] Filed Sept. 17,1969 PrimaryExaminer- David Schonberg [45] Patented Apr. 20, 1971 AssistantExanu'ner-Paul R. Miller [73] Assignee BellTelephone Laboratories,Incorporated Attorneys-R. .l.GuentherandArthurJ. Torsiglieri MurrayHill, NJ.

ABSTRACT: There is disclosed an arrangement of twocoordinate deflectorswithout the 90 polarization rotator [54] JB between the stages that waspreviously thought to be essential. g l 3 on This reduction in thenumber of optical parts is achieved by mm mutually rotating only theinclined prism interfaces in the [52] U.S.Cl 350/150, successive stages,while maintaining the crystalline axis 250/199, 350/157, 350/160orientations and electrode orientations the same in the two [51] lnt.ClG02! 3/00 stages. The new deflector is easier to align and scatters less[50] Fieldol Search 350/147, light than prior prism deflectors; and itwill be useful in 150,157,160,(Dig.2 D.L.D.);250/199,(Sci. multiple-paslight deflectors and optical time-division L.B.) multiplex communicationsystems.

11 2 26 CONE OF SOURCE ll J 2 J l PATENIEUAPRZOIQYI 3575.488

F/G' 26 coNE OF .2 51 L QEZII JA T S POLARIZATION H OF BEAM lg 8 1; .E.'T::'"T

POLARIZED I." I LIGHT W J SOURCE Il 1 23 FIG. 2 /2 I4 POLARIZATION OFBEAM FIG. 3 2

I I .n ANTI y' DIRECTION AND POLARIZATION I, y OF LIGHT E. A. aw i x I XgjR/E TRAMBARULO wwzww A T TORNE I SIMPLIFIED TWO-COORDINATEELECTRO-OPI'IC PRISM DEFLEC'IOR C ROSS-REFERENCE TO RELATED APPLICATIONThis application is filed concurrently with our related application Ser.No. 858,705, assigned to assignee hereof.

BACKGROUND OF Tl IE INVENTION This invention relates to electro-opticlight beam deflectors of the type employing compound electro-opticprisms with inclined interfaces.

All of the prior electro-optic light beam deflectors employing compoundelectro-optic prisms require a 90 polarization rotator between twostages which deflect in orthogonal coordinates. This 90 polarizationrotator causes additional optical absorption and scattering andcomplicates the alignment of the complete deflection apparatus.

SUMMARY OF THE INVENTION We have recognized that the 90 polarizationrotator is not basic to the compound electro-optic prism deflector andcan be eliminated by discarding the prior art convention of mutuallyrotating the entire assemblies of the respective stages by 90 and,instead, mutually rotating only the inclined prism interfaces in thesuccessive stages while maintaining the crystalline-axis orientationsand electrode orientations the same in the two stages. The beam isdeflected in the direction of the greatest mass of slow" material, as inprior compound electro-optic prism deflectors.

BRIEF DESCRIPTION OF THE DRAWING Further features and advantages of ourinvention will become apparent from the following detailed description,taken together with the drawing, in which:

FIG. I is a partially pictorial and partially schematic illustration ofan illustrative embodiment of our invention;

FIG. 2 is a modification of the essential portions of FIG. 1 showing anexploded view of the relationships of the compound electro-optic prismsand electrodes; and

FIG. 3 shows a pertinent diagram of the index ellipsoid for theelectro-optic material employed in the prisms.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT In the embodiment of FIG. I,it is illustratively desired to deflect a horizontally polarized lightbeam from a source 1 I in two orthogonal coordinates with a circularscan. It should be understood that this mode of operation is merelyillustrative, since it is the mode of operation preferred when thedeflection apparatus 12 is inserted between opposed reflectors of aninterferometer in order to provide multiple-pass light deflection. Suchdeflection is useful for multiplying the deflection angles in an opticaltime-division multiplex communication system of the type disclosed inthe copending patent application of Messrs. S. J. Buchsbaum and R.Kompfner, Ser. No. 631,301, filed Apr. 17, I967, and assigned to theassignee hereof now U. 5. Pat. No. 3,506,834. The deflection apparatus12 could be employed in any twocoordinate light beam deflection.including that required in random access optical memories.

The compound electro-optic prism deflection apparatus 12 includes thevertical deflection stage 13 and the horizontal deflection stage 14,illustratively driven in phase quadrature from a source I5 of sinusoidaldriving voltage of magnitude appropriate for the electro-optic materialemployed.

The vertical deflection stage 13 includes the electrodes 16 and I7 ontop and bottom surfaces of the compound electrooptic prism and furtherincludes the prism elements I8 and I9 which fill the space betweenelectrodes 16 and I7 and have an inclined interface which slopes fromthe electrode 16 to the electrode 17 in the direction of the lightpropagation. The normal to this interface lies in the vertical y-zplane. It will be noted that the plane of this interface is parallel tothe polarization of the light passing therethrough for the specific casein which the electro-optic material in both prism elements 18 and 19 ispotassium dihydrogen phosphate (KDP).

The horizontal deflection stage 14 includes the top and bottomelectrodes 21 and 22, respectively, and the compound electro-optic prismcomposed of prism elements 23 and 24 (in back), respectively. The prismelements 23 and 24 fill the space between electrodes 21 and 22 and havean inclined prism interface 25 running from the near left-hand comer tothe far right-hand comer in the direction of light beam propagationtherethrough. The normal to this interface lies in the horizontal plane,which can be called either the x-y plane or the x-y plane. It will benoted that this prism interface 25 is orthogonal to the plane of theelectrodes 21 and 22 and, for the specific case in which prism elements23 and 24 are KDP, is oblique to the polarization of the light beampassing therethrough. The polarization of the light in both deflectingstages 13 and I4 is horizontal; and no polarization rotator is employedbetween the stages I3 and I4.

For the illustrative case of circular scan deflection, the deflectionvoltage source I5 is connected directly between electrodes 16 and 17 tostage 13 and is connected between electrodes 21 and 22 of stage 14through a 90 phase shifter 26.

The crystalline axis orientation of the electro-optic prism elements instages I3 and I4 is more readily illustrated in the exploded view ofFIG. 2, to which reference is now made. In stage 13 the prism element 18has its z-axis (optic axis) oriented in the vertical direction. For thepurpose of defining the response of the index ellipsoid to an appliedelectric field we will consider the positive sense of its orientation,the 001 direction, to be upward, whereas the x-axis positiveorientation, the direction, is toward the right rear at 45 and they-axis positive orientation, the 010 direction, is toward the left rearat 45 with respect to the light propagation direction. The prism element19 has the opposed sense of positive orientation of the z-axis, that is,downward, whereas the positive sense of its x-axis is aligned with thepositive sense of the y-axis in element 18, and the positive sense ofits y-axis is aligned with the positive sense of the x-axis in element18. For the more general case of electro-optic materials, the relativeorientation of the prism elements can be obtained by considering that,when a uniform field is applied across the compound prism, both prismelements should provide relatively slow passage of the light beam towardthe same edge of the light beam, top or bottom, and relatively fastpassage of the light beam toward the opposite edge. Then the two prismelements yield an additive effect upon the deflection of the light beam.

In stage 14 the positive sense of the z-axis of the prism element 23 isoriented, following the convention established for stage 13, in thedownward sense and the positive sense of its y-axis is oriented at 45 tothe direction of beam propagation in the sense toward the right rear,the positive sense of its x-axis being oriented toward the left rear. Inthe prism element 24 the positive sense of the z-axis is orientedupward, the positive sense of the x-axis toward the right rear and thepositive sense of the y-axis toward the left rear. It will be noted thatthe prism elements 18 and 24 have identical orientations of all threecrystalline axes and, indeed, can be fashioned from a single piece ofelectro-optic material, since no polarization rotator is requiredtherebetween. Likewise, the prism elements 19 and 23 have identicalcrystalline axis orientations; but they are separated from each other byprism elements I8 and 24.

In the operation of the embodiment of FIG. 1 stage 13 will produce avertical deflection in a direct relation to the magnitude of drivevoltage applied between electrodes 16 and 17, as in prior electro-opticprism deflectors. We'will show that in conjunction with this verticaldeflection the horizontal deflection stage 14 will operate upon the samepolarization of the coherent light beam to produce a horizontaldeflection in a direct relation to the magnitude of the drive voltageapplied between the electrodes 21 and 22, which are parallel toelectrodes 16 and 17.

We will now give a detailed explanation of the deflection in the twostages for the specific case of KDP, for which the index ellipsoiddiagram in H6. 3 is pertinent.

Beam deflection in an electro-optic material is achieved by causing alinear change in velocity across the face of the beam, where the changein velocity due to an applied electric field, for any beam direction, orpolarization, is governed by the refractive index ellipsoid. For KDP,the ellipsoid equation is 1 2r,,E,YZ Z E ZX Zr E XY (1) where X Y, and Zare the components of refractive index in the [100], [010], and [OOl]directions in the crystal, taken as the .r, y and z directionsrespectively, n,, is the refractive index in the x and y directions, n,is the refractive index in the z direction, r and r, are theelectro-optic coefficients, and E E E, are the components of electricfield in the x, y, and 2: directions. The positive sense of each fieldis toward the point of more positive potential.

in the absence of an electric field, the last three terms of Equation(1) are zero. The ellipsoid then has a circular cross section in the xyplane and is shorter in the z direction, i.e., it is shaped like a doorknob, as indicated by the dotted lines in FIG. 3. When an electric fieldE, is applied in the positive 1 direction, the ellipsoid is elongatedalong the y axis (at 45 to the y-axis and compressed along the x axis(at 45 to the xaxis as shown by the solid lines in FIG. 3. There is nochange in the ellipsoid along the z-axis. Here the shape is very nearlythat of the initial door knob. Thus the change in the ellipsoidalsurface due to the electric field is geometrically simple, and theelectro-optic effects can be readily explained via the new ellipsoidwhich is shown in FIG. 3.

In all cases considered here, the beam is propagated along a principalaxis which remains fixed in direction; this is the y direction in FIG.3. As a result, the beams polarization can be resolved into componentsalong the other principal axes, and the velocity of each component issimply the velocity of light in free space divided by the refractiveindex in the direction of polarization.

As noted previously, the effect of +E on the initial ellipsoid is toperturb the circular cross section in the x-y plane into an ellipse asindicated in FIG. 3. The principal axes of the ellipse are along the xand y axes which are rotated 45 from the x and y axes. The index n, isless than its zero-field value of n and as a result, light propagated inthe y direction, with polarization x, is speeded up. For negative E, theprocess is reversed and the light is slowed down. From (I) it can beshown that For a typically strong electric field of E =3 l0 volts/meter(air breakdown), n r E =3.6X which is 1. Thus (2) can be written Itshould be noted that the opposed z-axis orientations, the 001directions, of the prism elements in each of the deflection stagesproduce a linear change in propagation speed across the face of thelight beam, provided the electric field is uniform throughout thatdeflection state. For a positive potential applied to the upperelectrode 16 an upward deflection occurs by virtue of the longer lengthof slow material at the top of the beam and the longer length of fastmaterial at the bottom. The crystal, polarization and electric fielddirections are consistent with FIG. 3.

To obtain horizontal deflection, the crystal, polarization and fielddirections for stage 14 are again consistent with FIG. 3, except thatthe vertical planar interface of the two prisms must be inclined to cutobliquely across the face of the beam. For a field applied with thepositive potential at the upper electrode 21, the greatest mass of slow"material will be on the left, and the greatest mass of fast" materialwill be on the right. There will again be obtained a linear change inspeed across the face of the beam, this time in the horizontaldirection.

Note that the beam polarization is always perpendicular to a zcrystalline axis; and the electric field isalways applied along a zcrystalline axis, both of which conditions are necessary for maximumdeflection sensitivity in KDP.

An analysis of the power requirements of our prism deflector is asfollows: In the prism deflector, the electric field is uniform, and thedrive voltage,

V,,,,-,,,,,, is: V,,,,-,,,,=2R,,E,

The capacitance, C energy stored, W,,,., and power dissipated, Purim arethose of a parallel plate capacitor:

Awhere (6 6 and tan 8 are the dielectric constant and loss tangentparallel to the optic axis, L is the length of the deflector, and w is211' times the drive frequency. The equivalent circuit for the deflectoris the capacity, C,,,,-,,,,, shunted y a resistance, R From Equations(5) and (6), and R=V/ 2P,

We claim:

1. Apparatus for deflecting the propagation direction of an incidentpolarized electromagnetic wave energy beam, said apparatus being of thetype comprising first and second compound electro-optic deflectionstages, each of said stages including two triangular electro-optic prismelements having like crystalline axes mutually opposed and a pair ofelectrodes disposed to apply an electric field along said opposed axes,the two prism elements forming an interface inclined to the direction ofpropagation, said apparatus being characterized in that the pairs ofelectrodes of said first and second stages are mutually parallel, andthe inclined interfaces of said first and second stages have normalslying in mutually orthogonal planes including the direction ofpropagation.

2. Apparatus according to claim 1 in which the first and seconddeflection stages are optically coupled to provide the same polarizationof the beam in both of said stages.

3. Apparatus according to claim 2 in which the second prism element ofthe first stage and the first prism element of the second stage in theorder of light passage therethrough have like crystalline-axisorientations.

1. Apparatus for deflecting the propagation direction of an incidentpolarized Electromagnetic wave energy beam, said apparatus being of thetype comprising first and second compound electro-optic deflectionstages, each of said stages including two triangular electro-optic prismelements having like crystalline axes mutually opposed and a pair ofelectrodes disposed to apply an electric field along said opposed axes,the two prism elements forming an interface inclined to the direction ofpropagation, said apparatus being characterized in that the pairs ofelectrodes of said first and second stages are mutually parallel, andthe inclined interfaces of said first and second stages have normalslying in mutually orthogonal planes including the direction ofpropagation.
 2. Apparatus according to claim 1 in which the first andsecond deflection stages are optically coupled to provide the samepolarization of the beam in both of said stages.
 3. Apparatus accordingto claim 2 in which the second prism element of the first stage and thefirst prism element of the second stage in the order of light passagetherethrough have like crystalline-axis orientations.